The xylenes, such as para-xylene, meta-xylene and ortho-xylene, can be important intermediates that find wide and varied application in chemical syntheses. Generally, para-xylene upon oxidation yields terephthalic acid that is used in the manufacture of synthetic textile fibers and resins. Meta-xylene can be used in the manufacture of plasticizers, azo dyes, and wood preservers. Generally, ortho-xylene is a feedstock for phthalic anhydride production.
Xylene isomers from catalytic reforming or other sources generally do not match demand proportions as chemical intermediates, and further comprise ethylbenzene, which can be difficult to separate or to convert. Typically, para-xylene is a major chemical intermediate with significant demand, but amounts to only 20-25% of a typical C8 aromatic stream. Adjustment of an isomer ratio to demand can be effected by combining xylene-isomer recovery, such as adsorption for para-xylene recovery, with isomerization to yield an additional quantity of the desired isomer. Typically, isomerization converts a non-equilibrium mixture of the xylene isomers that is lean in the desired xylene isomer to a mixture approaching equilibrium concentrations. It is also desirable to convert ethylbenzene to one or more xylenes while minimizing xylene loss. Moreover, other desired aromatic products, such as benzene, can be produced from such processes.
Various catalysts and processes have been developed to effect xylene isomerization. In selecting appropriate technology, it is desirable to run the isomerization process as close to equilibrium as practical in order to maximize the yield of the desired isomer as well as produce other desired aromatic products, such as benzene. However, greater isomerization activity can be associated with a greater production of undesired side products. Generally, it is desirable to have a catalyst with sufficient activity to isomerize xylenes yet not produce undesired side products that can lower the purity of desired products, such as para-xylene and benzene. Thus, a catalyst having a favorable balance of activity, selectivity, and stability would be beneficial.